1. Field of the Invention
The present invention relates to a droplet deposition apparatus. More specifically, the invention relates to circulating ink supply systems for use with the ink jet printing apparatus.
2. Description of the Related Art
Ink jet printing technology, due to its sheer simplicity (and its ability to dispense very small controlled droplets of ink) has found a great audience. Brochures, advertisement, fliers, business cards, labels are some application areas where this technology has been approved (applications that earlier relied on offset printing). The applications for this technology have expanded over the duration of its existence. From its beginning as a business documentation printing technology, ink jet (due to its vast appeal) has crossed over into the realm of large format printing, packaging and 3D prototyping. With the requirements within each of these industry segments becoming increasingly complex, ink jet technology has managed to keep pace and deliver on each occasion.
In traditional printing applications, ink jet printing technology is used for deposition of fine droplets of ink from minute nozzles onto a receiving medium in order to create a printed reproduction of an image. In a manufacturing environment, ink jet printing is used for microdeposition and coating in critical manufacturing processes. All of these applications have created a variety of ink jet processes and print head designs. The actuating mechanism for the development of droplets in the print head has evolved over a period of time and currently three main technologies drive ink jet printing. Ink jet print heads produce droplets either continuously or on demand. Continuous production means that the ink supply is pressurized sufficiently to create a continuous stream of ink drops exiting a nozzle. Drops are created for every possible pixel location on the recording medium since the pressurized ink supply cannot know beforehand when and where pixels will need to receive an ink drop. The many drops not needed for printing onto the recording medium (because of a ‘white’ pixel) are discarded in some fashion. Continuous ink jet print heads always need a gutter that can capture these discarded drops. Either the gutter drops or the print drops are deflected out of the continuous stream of drops emerging from the nozzle. The drop deflection force is usually electrostatic. ‘On demand’ differs from ‘continuous’ in that ink drops are only produced on demand by manipulating a physical process to momentarily overcome surface tension forces of the ink and emit a drop of ink or cluster of drops of ink from a nozzle. The ink supply is not sufficiently pressurized to form a continuous stream of ink drops. Instead, the ink is held in a nozzle, forming a meniscus. The ink remains in place unless some other force overcomes the surface tension forces that are inherent in the liquid. The most common approach is to suddenly raise the pressure on the ink, propelling it from the nozzle. One category of drop on demand ink jet print heads uses the physical phenomenon of electrostriction, a change in transducer dimension in response to an applied electrical field. Electrostriction is strongest in piezoelectric materials and hence these print heads are referred to as piezoelectric print heads. The very small dimensional change of piezoelectric material is harnessed over a large area to generate a volume change that is large enough to squeeze out a drop of ink from a small ink chamber. A piezoelectric print head includes a multitude of small ink chambers, arranged in an array, each having an individual nozzle and a percentage of transformable wall area to create the volume changes required to eject an ink drop from the nozzle. Another category of drop on demand ink jet print heads uses hot spot transducers, approximately the same size as an image pixel, that can be pulsed to boil a very thin sheath of liquid. The tremendous volume expansion of the liquid-to-vapour phase transition creates the same pressure pulse effect as does a huge area of piezoelectric transducer.
The present invention deals with the way ink is supplied to the ink chambers of drop on demand ink jet print heads and the conditioning of the ink for optimal operation in the ink jet print head.
In the prior art, ink circulation systems for ink jet printing apparatuses have been disclosed and have proven to be beneficial for avoiding ink deterioration while the ink is installed in the printing apparatus, e.g., due to segmentation of pigment particles. WO 2006/064040 (AGFA) 2006-06-22 disclosed such a circulating ink supply system for use with drop on demand ink jet print heads in production type printing equipment. The circulation ink supply system has a through-flow ink degassing unit mounted inline with the ink circulation, i.e., the ink flowing to the print heads also flows through the degassing unit. The inline degassing solves problems related to entrapped air in the ink supply path and problems related to rectified diffusion of insufficiently degassed ink in the ink chambers of the print head during the drop production process. An embodiment is disclosed wherein the principles of ink circulation and inline degassing are applied to an ink jet printing apparatus incorporating multiple print heads. The drawing illustrating this embodiment has been recaptured as FIG. 1 in this application. The driving force for ink circulation through the print heads and through the inline degassing unit is provided by a hydrostatic pressure difference Δp between the free ink surface in two different ink storage tanks. A hydrostatic pressure difference is, from a practical point of view, always limited and less suitable as a process variable to control an ink flow rate. Also, the actual flow rate in a hydrostatic driven ink circulation is dependent on the flow resistance in the flow path. This flow resistance may depend on the number of print heads connected, with the total length of tubing in the ink path, etc. Therefore the ink flow rate in the ink circulation system is limited in size and limited in controllability. On the other hand, the ink flow rate is an important parameter in controlling the efficiency of the inline degassing unit. The through-flow degassing unit discussed in WO 2006/064040 (AGFA) 2006-06-22 was said to operate best with an ink flow rate through the degassing unit of at least 1000 ml/hr, which is substantially higher than the ink flow rate created from the hydrostatic pressure difference Δp between the free ink surface in two ink storage tanks. To solve this problem, another embodiment that was disclosed includes a bypass path or shunt parallel to the main ink circulation path that serves the print head. A circulation pump creates an ink flow rate through the degassing unit that is substantially higher than the ink flow rate created from the hydrostatic pressure difference Δp. The bypass path acts as a shortcut return path for the degassed ink in excess of the ink required in the main ink circulation path. The shortcut return path therefore allows the flow rate through the degassing unit to be higher than the flow rate through the main ink circulation path, and therefore to better degas the ink circulating through the shortcut degassing circuit.
The technical problem of the prior art ink circulation and degassing system is that the main ink circulation path taps degassed ink from the shortcut degassing circuit, via controllable valves, at a low flow rate and stores the tapped ink in an intermediate storage tank before being used by the print head. The intermediate storage of ink is a potential source for re-introducing gas in the (previously degassed) ink. This process may be enhanced by the splashing of the ink in the intermediate storage tank during fast acceleration and deceleration of a traversing print head carriage on which the intermediate storage of ink may be mounted. Anyhow, every degassed ink that is exposed to air, e.g., in the intermediate storage tank, is gassed over time, e.g., during a standstill of the printing apparatus.